How do you say 'hello' in Japanese?

The correct response for saying 'hello' in Japanese is "Konnichiwa." This greeting is commonly used during the daytime and translates directly to 'good afternoon' in English. It reflects the cultural context of greetings in Japan, which varies depending on the time of day and the formality of the situation.

The other options are distinctly different phrases. "Arigatou" means 'thank you,' which is a term of gratitude not used for greeting. "O Genki Desu Ka" translates to 'How are you?' and is a follow-up greeting rather than an initial one. "Sumimasen" is often used to mean 'excuse me' or 'I'm sorry,' and is typically employed in situations of politeness or to get someone's attention, rather than as a standard greeting.